In the fabrication of integrated circuits, a number of well-established processes involve the application of ion beams to semiconductor wafers in vacuum. These processes include ion implantation, ion beam milling and reactive ion etching. In each instance, a beam of ions is generated in a source and is directed with varying degrees of acceleration toward a target wafer. Ion implantation has become a standard technique for introducing impurities into semiconductor wafers.
The energetic ions in an ion beam applied to a semiconductor wafer generate heat in the wafer This heat can become significant, depending on the energy level and current level of the ion beam and can result in uncontrolled diffusion of impurities beyond prescribed limits in the wafer. A more severe problem with heating is the degradation of patterned photoresist layers which are often applied to semiconductor wafers before processing and which have relatively low melting points.
Other semiconductor wafer processes such as ion etching, sputter deposition and etching, ion beam deposition, vacuum evaporation, plasma etching and chemical vapor deposition are performed in vacuum and may result in undesired heating of the wafer. In some instances, the process may require heat to be transferred to the wafer.
In commercial semiconductor processing, a major objective is to achieve a high throughput in terms of wafers processed per unit time. One way to achieve high throughput is by automation of the process for increased speed, reduced human handling of wafers, and more uniform and particulate-free devices. Another way to achieve high throughput in the case of an ion beam system is to use a relatively high current beam so that the desired process is completed in a shorter time. However, large amounts of heat are likely to be generated in the wafers being processed. Thus, it is necessary to provide wafer cooling in order to prevent the above-described detrimental effects of elevated temperatures.
A serious difficulty in effecting thermal transfer in vacuum is that thermal energy is blocked from conduction through a vacuum. The rate of thermal transfer from a semiconductor wafer by radiation is inadequate for most processes. Therefore, to achieve adequate rates of thermal transfer by conduction, it is necessary to physically contact the wafer with a thermally-conductive medium coupled to a heat sink. Although such a system is straightforward in principle, efficient wafer cooling for automated ion implantation systems has been difficult to achieve for a number of reasons.
Since the front of the wafer must be exposed for ion beam treatment, any clamping must be at the periphery or by centrifugal force. Direct solid-to-solid contact between a wafer and a flat metal heat sink is relatively ineffective since any bowing or irregularities in the back surface of the wafer result in regions where the wafer does not contact the heat sink surface. Furthermore, where contact does occur, microscopic voids in the wafer and heat sink surfaces result in actual physical contact occurring over only about five percent of the two surfaces. As a result, thermal transfer is poor. A contoured heat sink surface for optimizing conductive heat transfer between a wafer and a heat sink is disclosed in U.S. Pat. No. 4,535,835, issued Aug. 20, 1985 to Holden. The heat sink surface is contoured so as to impose a load that results in a uniform contact pressure distribution and a stress approaching the elastic limit of the wafer for a peripherally clamped wafer.
Other techniques for limiting wafer temperature during processing have included batch processing in which the incident ion beam is time-shared over a number of wafers so that the heating on any particular wafer is limited. A thermally-conductive fluid can be confined by a flexible diaphragm which contacts the back of the wafer as disclosed in U.S. Pat. Nos. 4,580,619 issued Apr. 8, 1986 to Aitken and 4,682,566 issued July 28, 1987 to Aitken.
The technique of gas conduction has also been utilized for wafer cooling in vacuum. Gas is introduced into a cavity behind a semiconductor wafer and effects thermal coupling between the wafer and a heat sink. Gas-assisted solid-to-solid thermal transfer with a semiconductor wafer is disclosed in U.S. Pat. No. 4,457,359 issued July 3, 1984 to Holden. A semiconductor wafer is clamped at its periphery onto a shaped platen. Gas under pressure is introduced into the microscopic void region between the platen and the wafer. The gas pressure approaches that of the preloading clamping pressure without any appreciable increase in the wafer-to-platen spacing, thereby reducing the thermal resistance. When gas conduction cooling is utilized, it is necessary to confine the gas to the region behind the wafer and to prevent escape of the gas into the vacuum chamber, since gas escaping into the vacuum chamber is likely to have detrimental effects on the process being performed.
Another prior art technique for thermal transfer in vacuum involves the use of a thermally-conductive polymer between a semiconductor wafer and a heat sink. A tacky, inert polymer film for providing thermal contact between a wafer and a heat sink is disclosed in U.S. Pat. No. 4,139,051 issued Feb. 13, 1979 to Jones et al. The polymer film disclosed by Jones et al has a sticky surface which is used to advantage to retain the wafer in position during processing. However, such a sticky surface is unacceptable in automated processing, wherein the wafer must easily be removed after ion beam treatment. The use of sticky surfaces in automated equipment often results in wafer breakage during wafer removal, or in an inability to remove the wafer from the sticky surface at all. Furthermore, particles, dust and other undesired materials tend to adhere to the sticky polymer surface and to contaminate subsequent wafers. In addition, cleaning of foreign matter from the sticky surface is difficult.
An automated wafer clamping mechanism utilizing a pliable thermally-conductive layer between a semiconductor wafer and a heat sink is disclosed in U.S. Pat. No. 4,282,924 issued Aug. 11, 1981 to Faretra. The wafer is clamped at its periphery to a convexly-curved platen having a layer of thermally-conductive silicone rubber on its surface. The Faretra apparatus has provided satisfactory thermal transfer under a variety of conditions. However, sticking of wafers to the silicone rubber surface has occasionally been a problem. To limit such sticking, relatively hard silicone rubbers have been utilized. However, the relatively hard silicone rubber is less effective with respect to thermal transfer, and intimate contact between the wafer and the convexly curved silicone rubber surface is not always achieved.
A technique for modifying the surface of a polymeric material utilizing ion implantation of selected ions is disclosed in British Patent Application No. 2,071,673A, published Sept. 23, 1981. However, the British publication contains no disclosure of a technique for preventing stickiness on polymer surfaces.
Imperfections and gas bubbles in the silicone rubber or other polymer layer can seriously degrade thermal transfer performance. When a gas bubble that is present during the molding process leaves a void on the surface of the silicone rubber layer, thermal transfer is reduced in the area of the void. When gas bubbles are located within the bulk of the silicone rubber layer, they can gradually outgas during vacuum processing, thereby causing a virtual leak in the vacuum chamber. It has been difficult to achieve a silicone rubber layer that is uniform and free of gas bubbles.
While the curved platens disclosed in U.S. Pat. Nos. 4,535,835 and 4,282,924 increase the area of contact between the wafer and the thermally-conductive surface, they introduce a spatial variation in angle between the incident ion beam and the wafer surface. In some processes such as ion implantation, angle-of-incidence variations can be a serious problem. The depth of penetration of incident ions is a function of incident angle because of the well-known channeling effect. Therefore, it is desirable in ion implantation to provide a constant angle-of-incidence between the ion beam and the wafer surface over the surface area of the semiconductor wafer.
It is a general object of the present invention to provide improved apparatus for effecting thermal transfer with a workpiece in vacuum.
It is another object of the present invention to provide a thermally-conductive polymer layer for effecting highly efficient thermal transfer between a semiconductor wafer and a heat sink or a heat source.
It is a further object of the present invention to provide a thermally-conductive polymer layer having a nonstick surface for thermal transfer with a semiconductor wafer.
It is still another object of the present invention to provide apparatus for thermal transfer with a semiconductor wafer wherein the wafer is maintained substantially flat during processing.
It is still another object of the present invention to provide apparatus for thermal transfer with a semiconductor wafer, including a thermally-conductive polymer layer having a thin surface film which substantially prevents adherence of the wafer.
It is a further object of the present invention to provide apparatus for effectively cooling a semiconductor wafer during processing in vacuum so as to permit a relatively high level of incident energy.
It is a further object of the present invention to provide apparatus for thermal transfer with a semiconductor wafer, which is simple in construction and low in cost.
It is another object of the present invention to provide apparatus for thermal transfer with a semiconductor wafer, including a thermally-conductive polymer layer having an extremely smooth surface for enhanced thermal transfer.
It is still another object of the present invention to provide apparatus for thermal transfer with a semiconductor wafer, including a thermally-conductive polymer layer that is very soft so as to conform to surface irregularities of the semiconductor wafer.
It is another object of the present invention to provide an improved method for effecting thermal transfer with a workpiece in vacuum.
It is another object of the present invention to provide a method of treating a thermally-conductive polymer layer used for thermal transfer with a semiconductor wafer to prevent adherence of the wafer to the polymer layer.
It is yet another object of the present invention to provide apparatus for thermal transfer with a semiconductor wafer including a thermally-conductive polymer layer having a thin surface film comprising a plurality of plates.